The present invention relates generally to magnetic resonance (MR) imaging and, more particularly, to a method and system of parallel imaging and image reconstruction that reduces the adverse effects of gradient nulls on a reconstructed image.
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B0), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but process about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a magnetic field (excitation field B1) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, or “longitudinal magnetization”, MZ, may be rotated, or “tipped”, into the x-y plane to produce a net transverse magnetic moment Mt. A signal is emitted by the excited spins after the excitation signal B1 is terminated and this signal may be received and processed to form an image.
When utilizing these signals to produce images, magnetic field gradients (Gx, Gy, and Gz) are employed. Typically, the region to be imaged is scanned by a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. The resulting set of received NMR signals are digitized and processed to reconstruct the image using one of many well known reconstruction techniques.
As described above, magnetic field gradients are employed for spatial localization during MR imaging. Ideally, the magnetic field gradients have linear spatial dependence (gradient fields) in three orthogonal directions. In practice, however, the gradient fields are generally only approximately linear within a limited volume. As a result, it is not uncommon for the gradient field at one or more locations to be negligibly small. In other words, the net magnetic field (the summation of the polarizing B0 magnetic field and the gradient field) at a given location may have nearly zero slope. These points of near-zero slope are commonly referred to as gradient nulls and can adversely affect image quality.
That is, if magnetization is present at such a location and the transmit RF field is sufficiently large at that location to excite the magnetization, the resulting signal can cause artifacts. Moreover, since the gradient field at a gradient null location provides almost no spatial encoding, the signal collapses upon itself within the image. As a result, the signal is of high intensity and can appear as a bright artifact in the reconstructed image.
To help negate the effects of these gradient nulls, gradient coils are typically constructed such that the gradient null locations lie outside the imaging volume (e.g. the center 48×48×48 cm3). However, it is possible for the signal from the gradient null to alias into the imaging volume with some phase or slice encoding processes. Moreover, if the signal does not directly alias into the imaging volume, system hardware instabilities, such as in the gradient or RF coils, eddy currents, or k-space modulation resulting from refocusing or different magnetization pathways in some echo train pulse sequences can cause ghosting of the intense signal in the phase or slice encoding directions. Moreover, this ghosted signal can manifest itself in the imaging volume even if the primary signal from the gradient null is not in the imaging volume.
While a number of MR imaging techniques can be impacted by gradient nulls, fast spin echo (FSE) imaging is particularly prone to gradient null ghosting. Fast spin echo imaging has a signal modulation that is inherent in the FSE pulse sequence even in the absence of system hardware instability that cause the aforementioned ghosting to occur in the reconstructed image. As a result, this gradient null artifact in FSE imaging is commonly referred to as a “FSE cusp artifact”.
This artifact has been shown to be particularly prevalent with spine imaging along the sagittal or coronal planes. Sagittal or coronal spine imaging is customarily carried out with a multiple-element receive coil or coil array with the coil elements or coils linearly arranged in a line in the superior-inferior direction. Phase encoding is typically defined in the superior-inferior direction for spine imaging to reduce ghosting from cerebral spinal fluid (CSV) flow. A gradient null typically lies either superior to or inferior of the imaging volume and, as a result, the intense signal associated therewith can alias directly into the imaging volume because of the superior-inferior phase encoding direction. Therefore, with spine imaging, the field-of-view (FOV) in the phase encoding direction is typically doubled relative to a desired FOV. Data from the excess FOV, which contains the gradient null signal, is then discarded. This larger-than-desired FOV is often referred to as a “no phase wrap FOV” as the FOV is purposely made larger than needed to prevent aliasing or phase wrap. Further, even with a “no phase wrap FOV” to prevent the signal from the gradient null from overlapping anatomy of interest, ghosting from the gradient null is possible within the desired FOV with FSE and similar pulse sequences.
A number of techniques have been developed to reduce the impact of gradient null ghosting or artifacts.
One proposed solution is centered on improving gradient coil design. Accordingly, the coil can be constructed such that RF transmit coil falloff and gradient coil locations are matched to one another. By moving the gradient null locations into an area of sufficiently small RF transmit energy, the transverse magnetization that is excited at the nulls is negligible. While it is recognized that improved gradient coil design would help reduce the impact of gradient nulls, such a solution would require retrofitting existing MR scanners with new hardware, which could be cost-prohibitive.
It has also been suggested that an RF blanket could be used to suppress the gradient null artifact through the absorption of RF energy near the gradient null to prevent the excitation of transverse magnetization. A drawback of an RF blanket, however, is that to be effective, the RF blanket must be precisely positioned within the imaging volume. Acquiring pre-scan knowledge of a gradient null location can be time consuming and negatively affect subject throughput. Moreover, it is possible for a gradient null to be near a subject's head. As such, to be effective, the subject's head may need to be overlaid with the blanket. Not only would such a placement of the blanket be uncomfortable for some subjects, but be impractical for near-head spine scans.
Chopping or phase cycling the RF excitation pulse has also been suggested as a solution. It has been suggested the gradient null artifact may be suppressed by chopping the 90 degree excitation pulse of a spin echo or FSE pulse sequence. However, such a solution ignores that the artifact signal, which will be excited by the RF excitation pulse, has the same polarity as the desired signal that is to be excited by the RF pulse. As such, the artifact will not be eliminated after the chopped signals are subtracted.
In another proposed solution, it has been suggested that slice gradient spoilers may be effective in suppressing the gradient null artifact when the artifact is created by RF pulses other than the excitation pulse, e.g. chemical or spatial saturation pulses, refocusing pulses, or fast recovery flip-up pulses. Such a solution would only effective if the gradient field from the spoiler gradient is sufficiently large at the null location to provide substantial dephasing over a voxel (volume element). As a practicality, however, the gradient null locations along the three gradient axes are typically relatively close together because all three gradient coils have similar spatial coverage or FOVs. Thus, the slice select gradient has relatively little slope at the location where the phase encoding gradient has a null. As such, it is recognized that spoilers on any axis would be ineffective at suppressing the signal from the gradient null.
Another proposed solution involves toggling the polarity of the phase encoding gradient so to remove or reduce the gradient null artifact by shifting the gradient null location. Such an approach is believed impractical. That is, the optimal polarity of the gradient is unknown in advance. Moreover, it is possible for the gradient null artifact to be just as severe at the new location. In other words, there is no guarantee that the artifact will be less severe at the new gradient null location than at the original location.
It has also been proposed that the phase and frequency directions can be swapped in cases where the gradient null artifact is severe. It is believed that this can be an effective management strategy for gradient null artifact reduction. However, this approach may not be practical for many applications because of motion artifacts. Additionally, it has been suggested that increasing the FOV can be helpful in preventing signal from a gradient null from aliasing into the imaging volume. However, simply increasing the FOV does not prevent ghosting of the signal from overlaying the reconstructed image. Further, simply repositioning the subject to shift the relative location of the artifact is believed ineffective and, moreover, generally inconvenient.
It has also been suggested that parallel imaging can be an effective approach for artifact reduction. Such an approach assumes that the ghosts from the gradient null signal are differently weighted (but otherwise exact), replicated copies of the true magnetization. However, this assumption is inapplicable to artifacts arising from gradient nulls. In other words, parallel imaging has been shown to be capable of reducing conventional ghosting in echo train pulses where the ghosting arises from signal within the imaging volume, not gradient nulls.
In another suggested solution, images from a multiple coil array are combined to cancel motion and flow artifacts. With this technique, the region where signal is to be canceled must be manually selected. The resulting signal cancellation removes artifacts from the entire image. However, this technique is not practical for those instances where the signal from the gradient null (the signal that would be canceled) is aliased into the imaging volume.
Further, in another technique, it has been suggested that parallel imaging may be used to force data consistency through the elimination of artifacts of several types. While gradient null artifacts are not specifically addressed with this approach; nevertheless, the technique is complex and computationally intensive.
It would therefore be desirable to have a technique for reducing the adverse effects of gradient nulls in a reconstruction image that is practical in its application and applicable for those instances when the signal from a gradient null is aliased into the imaging volume that does not require existing coil retrofitting.